1. FIELD OF THE INVENTION
The present invention relates generally to high technology ceramic materials and more specifically, to the manufacture of ceramic tiles of boron carbide having particularly advantageous application as ceramic armor.
2. PRIOR ART
Boron carbide is the lightest and hardest ceramic known to man. It has a mass of 2.5 grams per cubic centimeter and a hardness of 3,000 Knoop.sub.100 Kilograms per square millimeter. As a result of these advantageous characteristics, namely, its low weight and extreme hardness, boron carbide has become well-known for its advantageous application as an armor material. Such material is normally provided in the form of rectangularly shaped tiles for protection of aircraft seats, hydraulics, cables, electronics, fuel lines and in other shapes for rotor hub assemblies in helicopters and so on. Typically, the ceramic material is structured with a layer of Kevlar on one side and a nylon spall shield on the other to provide a combination with the maximum stopping power for various threats comprising various calibers in the form of shells, bullets and the like.
Typically, such boron carbide tiles have dimensions of 8-12 inches along one dimension and 8-18 inches along the other. They are typically in the range of 1/4 inch to 1/2 inch in thickness and are generally rectangular in shape with precision straight edges to render each tile geometrically compatible with adjacent tiles which in combination comprise the armor shield for a variety of applications including, for example, seat and airframe armor in the most advanced attack helicopters.
Typically, ceramics such as boron carbide are manufactured using a hot pressing process in which the ceramic is initially in a powder form and is compacted into a billet substantially identical to the ultimate ceramic shape desired. The precompacted billet is then loaded in a die and hot pressed. Hot pressing is a process in which high pressure is applied to the billet at extremely high temperatures. By way of example, it is not unusual in hot pressing manufacturing process of boron carbide ceramic tiles to utilize temperatures above 2,000 degrees Centigrade and pressures above 5,000 psi. The hot pressing process causes the boron carbide to become sintered and ultimately, upon cooling, the finished ceramic tile is an extremely hard ceramic material and therefore an ideal material for armor protection purposes.
Unfortunately, the incredible hardness of boron carbide ceramic also renders it disadvantageous from a manufacturing point of view. More specifically, in order to provide access to a structure, for example, a helicopter seat, it is often necessary that holes be provided in the tile to provide means for securing accessories by way of a plurality of bolts. However drilling holes in a material as hard as boron carbide ceramic is no easy task. Conventional machine drilling tools are no match for the harder ceramic material. Specially designed diamond drills are too expensive and the process of drilling too time consuming to provide a feasible and cost-saving process for drilling holes in boron carbide ceramics. As a result, one method of overcoming this manufacturing difficulty in the production of boron carbide ceramic armor tiles has been to replace the drilling process with a cutting process. This prior art method of overcoming the difficulties of drilling through boron carbide ceramic will be described hereinafter in more detail in conjunction with the detailed description of the invention. However, as it will be hereinafter more fully understood, the cutting process also has significant disadvantages which detract from this alternate manufacturing technique and make it highly desirable to find an alternative. For example, the substitute cutting process for providing holes in ceramic armor tiles is also a costly process because it involves significant additional man-hours but more importantly, it detracts significantly from the effectiveness of the tile material in terms of its threat-stopping capabilities. These relative disadvantages of the prior art process will be discussed hereinafter in more detail.